The present invention relates to a non-contact type liquid level sensor and a non-contact type liquid level detecting method and, more particularly, to a non-contact type liquid level sensor and a non-contact type liquid level detecting method which are adapted to write output values which correspond, respectively, to an uppermost position and a lowermost position of the float, which are set in advance, to a memory and then to detect with high accuracy a liquid level corresponding to the position of a float by making use of data within the memory.
In a liquid level sensor which is installed in an automotive fuel tank of an automobile for detecting the volume of stored liquid fuel therein, the motion of a float arm adapted to rotate according to the movement of a float is grasped as a change in electric resistance value so as to output a voltage according to the variation, so that the resulting fuel level is indicated on a measuring instrument.
In this liquid level sensor, a contact on a contact piece mounted on an arm holder which supports a float arm is brought into slide contact with a resistor on an insulating substrate provided on a frame (a casing). Consequently, a voltage value obtained according to the motion of the float arm or the slide contact position of the contact on the contact piece corresponds to the liquid level (for example, refer to Patent Document 1).
In the non-contact type liquid level sensor of this type, the contact and the resistor are oxidized or partially worn out to thereby call for a variation in resistance value or constitute a cause for generation of noise, eventually causing a problem that the detection accuracy of the liquid level is gradually deteriorated.
To cope with this, there has been proposed a non-contact type liquid level sensor which uses a magnetoelectric converting element such as a Hall-effect element. The non-contact type liquid level sensor of this kind is used as a level sensor for monitoring the volume of liquid fuel of an automobile, for example. In this non-contact type liquid level sensor, a change in magnetic field of a magnet adapted to rotate according to the movement of a float is detected by the magnetoelectric converting element and a magnetoelectric conversion signal (an electric signal) according to the change in magnetic field is outputted. In addition, the liquid level is displayed on a measuring instrument based on the magnetoelectric conversion signal.
In addition, in recent years, a Hall-effect IC to which a function to perform processes for correcting and amplifying magnetoelectric conversion outputs is added is used as the magnetoelectric converting element. This Hall-effect IC writes a magnetoelectric conversion output value corresponding to the rotational angle of a float arm being provided with a float to a memory and further makes a liquid level corresponding to the output value be displayed on a measuring instrument.
Consequently, by writing an uppermost position and a lowermost position which constitute limits to which the float can move upwards and downwards, respectively, to the memory of the Hall-effect IC, an output value corresponding to an actually measured position of the float between the uppermost position F and the lowermost position E thereof can be outputted as a liquid level, as shown in FIG. 3. As this occurs, the uppermost position F and the lowermost position E are determined by stoppers which restrict the rotational quantity of a float arm being provided with a float at a distal end thereof.
FIG. 4 is a conceptual drawing showing the construction of a non-contact type liquid level sensor 11 having a Hall-effect IC such as the one described above. This non-contact type liquid level sensor 11 takes a form in which one end of a float arm 13 is rotatably supported substantially at a central portion of a casing 12, which constitutes a sensor main body.
This float arm 13 is made of a metal rod which is bent to be formed into an L-like shape as a whole, and the float arm 13 so bent is further bent to be formed into a V-like shape at a predetermined portion from the one end thereof, so that a locking rod portion 13a is formed which is adapted to be brought into face contact with stoppers 14, 15 as appropriate.
A float 16 is fixed to a distal end (the other end) of the float arm 13 with a push nut or the like. The float is made of a material exhibiting a buoyant force relative to a liquid to be measured.
The stoppers 14, 15 are arranged so as to form a shape in which an upper isosceles triangle and a lower isosceles triangle are brought into abutment with each other at vertexes thereof, and positions where the locking rod portion 13a is brought into contact with sides a, b of the stoppers 14, 15 which face the locking rod portion 13a are made to be an uppermost position F and a lowermost position E of the float 16, respectively.
On the other hand, provided in the casing 12 as shown in FIG. 5 are a rotational shaft 17 which holds (fixes) the one end of the float arm 13, a magnet 18 provided integrally around an outer circumference of the rotational shaft 17, a pair of stators 19 which are disposed on a perimeter of the magnet 18 and an Hall-effect IC 20 interposed between the respective stators 19 (in a gap between the stators).
In this liquid level sensor 11, the float arm 13, which is adapted to rotate as the float 16 fluctuates, rotates the rotational shaft 17, which is fixed thereto, and the magnet 18. Due to this, the Hall-effect IC 20 detects a change in magnetic flux which follows a magnetizing pattern of the magnet 18 and outputs a corresponding electric signal.
The Hall-effect IC obtains a moving position of the float 16 within an area between the uppermost position and the lowermost position that have been written to the memory, or a rotational position of the float arm 13, and outputs an electric signal which corresponds to the change in magnetic flux so detected.
Patent Document 1: JP-A-2004-20538
Since the related non-contact type liquid level sensor is constructed as has been described heretofore, in the event that there are dispersions in dimension with respect to the constructions and mounting of the stoppers 14, 15, which restrict the quantity of movement (rotation) of the float arm 13, and the float arm 13, or dispersions with respect to input and output characteristics of the Hall-effect IC 20, output values from the Hall-effect IC become inaccurate.
Namely, when writing output values (a straight line A) which correspond to the uppermost position F and the lowermost position E as shown in FIG. 6, due to dispersions in dimension of the stoppers 14, 15, output values indicated by a straight line B are written, in reality, as shown in FIG. 6. Consequently, an output value obtained for each float position based on the dispersed output values or a liquid level value which is finally outputted as a measured value and an indicated value becomes unreliable.